Sarbella does have a decent physical attack, but there are also a few tricks up his sleeve; he's a pretty average monster that can be taken out with two or three physical attacks, but he has Roar, which will increase his physical attack, and he'll also occasionally counter your attacks with one of his own. Another thing that may be worth noting, is that he's a fire-based monster, and first appears at a time when you do not yet have the Aqua spell. This means Seth and Kaim should focus on him while Jansen should apply his efforts to another enemy, if possible.